A soldering method is widely used as so called a conductive jointing method of establishing electrical and physical connection between two conductive members. For example, when a semiconductor chip part is mounted on a lead frame to manufacture a semiconductor device, solder jointing has been widely used for conductive jointing of a rear-surface electrode layer provided on the rear-surface of a semiconductor chip and a pad portion on the surface of the lead frame. Specifically, a tin alloy solder material, particularly a tin-lead alloy solder material, is a low temperature melting alloy, and has been widely used as a conductive joint member in die-mounting of a semiconductor chip. For example, in a semiconductor device in which the surface density of a current passing through the surface of a rear-surface electrode that is conductively jointed is high, such as a semiconductor laser, the resistance of a conductive joint portion itself is a main component of a series resistance, and the conductive joint member itself is required to have a low volume specific resistivity. On the other hand, in a semiconductor device having a configuration in which a region of operation for the device is formed on the surface of the semiconductor chip, and a predetermined bias is applied to the rear-surface electrode itself for keeping constant the potential of a semiconductor substrate used therein, e.g. the rear-surface electrode is grounded, but essentially no current passes through the surface of the rear-surface electrode, the magnitude of the volume specific resistivity of the conductive joint member itself is not considered particularly important. However, it is required to lower a heat resistance in the conductive joint portion in the process of diffusing heat to the lead frame by the route passing away the semiconductor substrate, the rear-surface electrode provided on the rear-surface thereof, and the conductive joint portion in order to dissipate heat generated in the device operation region provided on the surface of the semiconductor chip and thus to prevent a rise in operation temperature of the device itself.
When conductive jointing using tin-lead alloy solder is employed in a step of die-mounting this type of semiconductor chip, the heating temperature for solder melting ranges within 250° C. to 300° C. or the like, and a thermal effect on the device operation region provided on the surface of the semiconductor chip by such a heating temperature is quite limited. In a process for manufacturing the semiconductor device, a step requiring heating of the entire lead frame having the semiconductor chip mounted thereon, such as a wire bonding step, is carried out after the die-mounting step, but for avoiding damage originating from the above-mentioned heat history to such a conductive joint portion comprised of a tin-lead alloy soldered layer, a solder material having a high ratio of lead, the melting temperature of which is 250° C. or higher, is put to good use for tin-lead alloy solder used therein.
For tin-lead alloy solder functionally suitable as a conductive joint member like this, contained lead itself exhibits high toxicity when it is converted into various kinds of lead compounds, for instance lead oxides, and therefore avoiding use thereof is an essential problem to be solved in future. Currently, instead of tin-lead alloy solder, tin alloy solder containing no lead, so called lead free solder is being developed and progressively used. Most of lead free solder that is currently developed is low temperature melting type of lead free solder comprising tin as a main component, and unfortunately lead free solder having a melting temperature set to be within the range of 250° C. to 300° C., which is fit for the above-mentioned die-mounting of the semiconductor chip, is still under development in the present stage.
In addition to the conductive jointing method using tin-based alloy solder, a conductive jointing method using a conductive paste type of conductive adhesive agent is also used for a device for which the series resistance originating from the conductive joint portion forming an electrical connection to the rear-surface electrode of the semiconductor chip, and the heat resistance are not necessarily required to be low. The conductive paste type of conductive adhesive agent is paste where, as a metal filler (metallic filling material), a metal powder that is used for a conductive medium is uniformly dispersed in a thermosetting resin component, and the resin component is thermally cured by a cure treatment, and the cured resin is used as a binder resin to establish close physical contact between the metal fillers. This layer of cured product constitutes a conductive layer as a whole by close physical contact between metal fillers contained therein, and an electrical channel at an interface between the metal surface to be jointed and the layer of cured product is formed also due to physical contact between the metal filler being densely present on the surface of the layer of cured product and the metal surface to be jointed. By taking advantage of an adhesive property of the binder resin forming the layer of cured product, bonding and fixation of the layer of cured product to the metal surface to be jointed is achieved, and a current flow channel through physical contact between the metal filler and the metal surface to be jointed is maintained. The current flow channel constructed by a closely formed network of mutually contacting metal fillers also functions as a channel of heat flow, and resultantly the layer of cured product itself exhibits a considerably good thermal conductivity.
A proposal has been made for improving the electrical conductivity and thermal conductivity performance of the resultant layer of cured product itself compared with that made of a conventional conductive metal paste using a metal filler having an average particle diameter of about several micrometers as a conductive medium. A hybrid type conductive metal paste exhibiting a markedly improved electrical conductivity property has been developed by, for example, blending a small amount of ultrafine metal particles having an average particle diameter of 100 nm or less, in particular 20 nm or less, in addition to the metal powder having an average particle diameter of about several micrometers, as the conductive medium used in the conductive metal paste, whereby regions filled with ultrafine metal particles in narrow gaps between adjacent metal powders are provided, in addition to the closely formed network of mutually contacting metal powders, and sintering ultrafine metal particles in the filled regions at a low temperature when carrying out a heating treatment (see International Publication No. 02/035554 Pamphlet). This hybrid type conductive metal paste has been developed for the purpose of producing a fine wiring pattern using mainly the conductive metal paste, but the structure, in which narrow gaps between adjacent metal powders are filled with a low-temperature sintered body made of ultrafine metal particles to impart an excellent electrical conductivity property to the layer of cured product itself, also gives an excellent thermal conductivity property thereto.